The following example adds a rectangle to slide one in the active presentation and sets its fill foreground color to the first extra color. If there hasn't been at least one extra color defined for the presentation, this example will fail.
With ActivePresentation
Set rect = .Slides(1).Shapes _
.AddShape(msoShapeRectangle, 50, 50, 100, 200)
rect.Fill.ForeColor.RGB = .ExtraColors(1)
End With